I stumbled on a similar problem with the space navigator and the new FP and I think I found the cause (not sure if it's on itoo's or on 3dcnx's side)
I had a scene in which all the trees were set to "auto assign to active view" - that way they where facing each camera at render time, while allowing me for a smooth navigation with the Space Navigator in perspective view - forest pack wasn't considering the 3dxstudio perspective as an active view and therefore the trees didn't move (orient themselves).
Since updating to 4.0.3, my viewport became really slow with lots of lags, even when selecting objects. I didnt' realize at first it was related to the forest trees since they were on a turned off layer. Since max's behaviour was driving me nuts, I starded to test everything and got the the trees - now they were trying to follow the active view - normal perspective with SpaceNav navigation - and that took a looooong time. Apparently they tried to do the same thing even when the layer was off. Anyway, in the end I turned the "auto assign to active view" off and the perspective navigation smothness returned to normal.
Unfortunately this is only a workaround, and since we might be a minority using both Forest Pack and a 3d connexion mouse...we might never know what actually happened.
Hope it helps,